5870 + 5850 Xfire? Or 5850 + 5850?

This is going to be a long winded post, but I'll try and be as short as I can about explaining my situation. First off, my specs:

AMD FX 8350 @ 4.6 (water cooled) *

Biostar TA970 (water cooled)

8GB of G.Skill Sniper at 1866 11-11-11-30 *

500Watt Modular Cool Master (80 Plus Certified)

64GB A data SSD + some mechanicals

Sapphire 5870 @ 900/1250 (water cooled)

*Still working on getting my memory as fast as possible, so these frequencies will change

My buddy has an OCZ GameXStream 700watt PSU that is unneccesary for his system, so we were planning on trading PSU's, which will open some doors for me. Mainly in terms of Crossfire, which will be nice.

I'm managing well with the single 5870, it does pretty well everything I would like it to do, but it's always nice to have that little extra head room because...hey why get 40FPS when you could get 55-60 right? So I was debating on whether or not getting a used 5850 to complement the 5870, and allow me to do eyefinity down the road, or just enjoy some extra frames. But this is where I'm confused.

I was reading up on Legit Reviews where they did the exact same GPU setup as what I was looking at, and here are their numbers: http://www.legitreviews.com/article/1107/6/

In short, two 5850's out perform a 5870 + 5850 setup, and consume almost 40W less!

What would you guys recommend? Should I look into flashing a new BIOS onto my 5870, turning it into a "super 5850" (as NCspecV81 from overclock.net put it when he was benchmarking 4 5870's) or run the 5870 + 5850 as they are, and hope that driver quality since then has improved.

Thanks for any input, I'm curious as to what you guys have to say about mixing GPU's for crossfire. (It's also a neat project, even though I have no plans to LC the second GPU. As I don't have another MCW60)

Basically, when you crossfire a 5870 and 5850, the 5870 will drop to the speed of the 5850. You should use AMD drivers and overclock yourself, and also maybe consider selling the 5870 and buying one single higher end card.

Furthermore, how is that OC on the cpu stable on that Mobo?

I understand that the GPU clocks will mesh. As in the 5870 will run at 725Mhz. However I use MSI Afterburner, and can force an OC. I just purchased a used Asus 5850 for $90 on Ebay. The one with the aftermarket cooler. I'm excited to see how it runs, as the seller has already shipped the card.

To answer your question, the board is solid, no issues. Vdroop is large, (like 0.05v) but other than that, I have no complaints. Why do you ask?

I ask because biostar in my experience only, I may just be unlucky, don't have stable boards. There are also few powerphases on that board